The present invention relates generally to an apparatus and method for cutting parts from a moving continuous stock, and more particularly, to an apparatus and method of accurate and reproducible cutting of a moving, continuous metal stock of light cross section and gauge.
Modern roll forming machines are capable of producing a continuously moving stock at high linear speeds having from very thick to very light gauge in a variety of cross sectional configurations. As the roll formed product is delivered from the forming machinery, it is conventionally necessary to sever the stock workpiece into parts having predetermined lengths by an associated cutting device. While roll forming machines can be employed to shape many different products, a problem of accurately cutting a stock piece which is easily deformable by an associated cutting device has not heretofore been satisfactorily solved. When light gauge strip material is roll formed, it is often desirable to pre-punch finish work openings into the strip of material prior to roll forming which remain in the severed part. Consequently, it is often necessary to accurately and reproducibly cut roll formed stock which has finish work openings in such a manner as to maintain the accuracy of the spacing of the finish work in the severed part.
Several conventional methods and apparatus are known and employed in the field of cutting roll formed stock. One method employs a cut-off press coupled to a positive engagement length stop which is physically engaged by the end of the moving stock piece. The moving stock piece must have a heavy gauge or sufficient lateral and transverse strength to be able to push the cut-off press directly along with the stock. At a preselected length from the edge of the stock, a limit switch is tripped causing the moving press to cut transversely through the stock piece. The press and length stop assembly is returned to its starting point and will again be engaged and pushed by the moving workpiece. This method is unsatisfactory for light cross sectional, light gauge roll formed stock which will easily deform or buckle by the back pressure or shock of engaging a limit stop and pushing a press bed. When the press bed is pre-accelerated at a slower speed than the speed of the roll formed product, the stock piece will also be deformed by encountering the shock and back pressure of having to push even a pre-accelerated press.
Another method employs a flying trip switch in place of a limit stop, at a predetermined spacing from either a stationary or pre-accelerated press bed. However, embodiments employing this method are not accurate to close tolerances. Electrical and mechanical variations cause response time delays preventing reproducability of part length. Any finish work, such as notching or punching, must be performed on the part after it has been roll formed and cut by this method.
Other methods are known which eliminate positive engagement with a limit stop and employ an accelerated press bed. A complicated electro-hydraulic system is employed to monitor the line speed in an attempt to provide an accurate end cut-off press and compensate for the variations in the line speed of the roll formed product. Those systems are unsatisfactorily subject to accumulated errors and inaccurate parts as a result of variations in mechanical and electrical factors which cannot be controlled such as press response time fluctuations.
In view of the prior art, a need currently exists for an improved method and apparatus for end-cutting accurate and reproducable part lengths from an easily deformable roll formed stock of light cross section and/or light gauge.